1. Field of the Invention
The present invention relates to a discharge lamp, and more particularly, to a metal halide lamp of a type equipped with a light filter for removing incident light rays of certain wavelengths.
2. Description of the Prior Art
A metal halide lamp device has a discharge bulb which contains therein mercury and metal halides such as sodium halide, scandium halide and tin halide emitting light rays therefrom. During start-up period with several tens of seconds after the discharge bulb has been energized, light rays radiating from the discharge bulb have a poor color rendition. That is, light rays radiating from the discharge bulb have a bluish color which is characteristic of radiation from mercury. The reason for this is that the interior of the discharge bulb has a relatively low temperature during the start-up period so that light rays are predominantly radiated from mercury. As is shown by a solid line "a" of FIG. 1, a relative spectral distribution of light rays radiating from the discharge bulb during the start-up period is confined to four discrete wavelengths, i.e. 405 nm, 436 nm, 546 nm and 578 nm wavelengths, in the visible portion (380 nm to 720 nm) of the spectrum. It is noted that a plurality of monochromatic light rays which are 405 nm, 436 nm, 546 nm and 578 nm wavelength have colors of violet, blue, green and yellow, respectively. This combination of the four wavelengths makes light rays bluish in color during the start-up period.
In a steady state of the discharge bulb, i.e. after the start-up period of the same, as the bulb warms up, light rays are radiated from the above-mentioned metal halide molecules or from metals of the same. As is shown by a dotted line "b" of FIG. 1, light rays radiating from the discharge bulb during the steady state are composed of relatively many different wavelengths, i.e. a relatively continuous spectrum as compared with the spectrum of light rays during the start-up period. Therefore, light rays under the steady state of the discharge bulb which are composed of a combination of different wavelengths, have a superior color rendition, i.e. white or at least whitish in color.
In view of the above-mentioned characteristics of the metal halide lamp, there are proposals to improve color rendition of the discharge bulb, i.e. to give off light rays of a white color during its start-up period.
For example, Japanese Patent First Publication Hei-2-256153 discloses a metal halide lamp device which is equipped with a multi-layer interference film or a high-pass filter for radiating light rays of a low color temperature and for improving color rendition. However, this device is still unsatisfactory in improving color rendition during the start-up period. In fact, this device radiates light rays which are greenish in color, because the interference filter is designed to remove light rays of wavelength shorter than about 500 nm, and thus the filter can not sufficiently remove light rays of 546 nm (green) wavelength.
When the metal halide lamp device is used as an automotive headlamp, it is turned on and off frequently in some cases. That is, it is repeatedly and intermittently kept in the start-up period. Thus, if color rendition of the metal halide lamp device is poor, it makes the driver's eyes tired.